1. Field of the Invention
The present invention relates to audio amplifiers, and in particular to digital class-D amplifiers.
2. Background Art
Conventional digital class-D amplifiers typically include a PWM stage, an output stage, and a demodulation stage. The modulation stage usually employs pulse-width modulation to generate a digital pulse-width modulated (PWM) signal, which comprises two values: high and low. The PWM signal results from a comparison operation that compares an input signal and a high frequency ramping or triangular carrier waveform. The PWM signal contains the frequency content of both the input signal and the carrier waveform. Within each period of the carrier, the duty ratio of the PWM signal is proportional to the amplitude of the input signal. The output stage typically includes power transistors, for example, metal-oxide-semiconductor-field-effect transistors (MOSFETs) that are driven by the PWM signal. The output stage switches between positive and negative power supplies to produce a train of voltage pulses that represents an amplified version of the input signal. In the demodulation stage, the amplified PWM pulse train may be passed through a low-pass filter to recover the input signal and filter out the carrier signal.
Digital class-D amplifiers are popular in applications that require a compact size and low power consumption. This is because digital class-D amplifiers have higher efficiencies due to the power transistors in comparison to linear amplifiers, such as class-A, B or AB amplifiers. In other words, most of the power supplied to a digital class-D amplifier is delivered to the load. Thus, the digital class-D amplifier dissipates much less power than a linear amplifier and therefore does not require the cooling measures (e.g., heat sinks) that the linear amplifier requires. Digital Class-D amplifiers may therefore be favored over linear amplifiers because of higher efficiencies, lower power consumption, and smaller sizes.
However, conventional digital class-D amplifiers may suffer from poor power supply rejection and poor distortion. Sometimes a feedback loop is used to decrease noise and distortion in a digital class-D amplifier. However, the output stage is not included in the feedback loop of the traditional digital class-D amplifier. Thus, the noise and distortion introduced in the output stage is not attenuated. Conventional digital class-D amplifiers may also suffer from output stage gain variation due to the power supply variation.
The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears.